(1) Field of the Invention
The invention relates generally to sonar arrays with acoustic discontinuities, and, more particularly, to increasing the directivity gain of a sonar array aperture by measuring particle motion of Bragg scattered longitudinal waves induced in the array as a result of incident waves that encounter the array.
(2) Description of the Prior Art
It is often desirable to determine the direction from which an acoustic signal arrives with as much accuracy as possible. Accordingly, increased gain in array directivity, especially at very low frequencies, is desired and sought by sonar designers.
The following patents and publication show various types of acoustic arrays with coatings and discontinuities that have been utilized in the past to solve various problems.
U.S. Pat. No. 5,808,970, to Nussbaum, et al., issued Sep. 15, 1998, discloses a sonar array uses multiple acoustically transparent layers. One layer is a linear array of acoustic sensors that is substantially acoustically transparent. Another layer is an acoustically transparent wiring assembly that provides electrical connection to each acoustic sensor. A third acoustically transparent layer is a planar array of signal processing circuits coupled to the wiring assembly for processing electrical signals generated by the acoustic sensors. Each signal processing circuit resides within an area that is in geometric correspondence with a respective one acoustic sensor. Each signal processing circuit can include a preamplifier, an analog-to-digital converter and a digital multiplexer.
U.S. Pat. No. 6,262,944, to Meyer, et al., issued Jul. 17, 2001, discloses an acoustic array including a plurality of acoustic sensors positioned at predetermined telemetry points for measuring the acoustic energy from the surrounding environment. The acoustic sensors are each positioned within a fluid-filled pocket in the acoustic array, wherein each the various fluid-filled pockets are separated from each other by a section of solid fill buoyant material. The sections of solid fill material control the buoyancy of the acoustic array. The acoustic array is surrounded by a longitudinally extending outer hosewall which encloses the acoustic array and forms the fluid-filled pockets between the sections of solid fill material. A strength member is extended longitudinally throughout the length of the acoustic array to provide a load bearing mechanism for distributing longitudinal tensile loads applied to the acoustic array, while support spacers are situated within the acoustic array to provide radial support for the acoustic array. The acoustic array is formed of a lightweight and electrically passive structure which allows water to be used as the fill fluid for the pockets. Using water as the fill fluid provides an environmentally-safe fill fluid having buoyancy characteristics which are not significantly altered by its mixture with water from the surrounding environment should a rupture in the hosewall occur.
U.S. Pat. No. 7,206,258, to Fisher, et al., issued Apr. 17, 2007, discloses an array of pressure sensors and motion sensors contained in a layered material system that includes an acoustically compliant layer and an acoustically transparent layer. The compliant layer (which vibrates in accordance with acoustical influence thereupon) is the foundation for both sensor types and is the vibratory medium for motion sensing. The transparent layer is the matrix for both sensor types and is the window permitting sound waves to reach the pressure sensors (which sense pressure of the sound waves) and the compliant layer (the vibration of which is sensed by the motion sensors). The compliant layer's exposed surface can be attached to a structure's exterior for passive sonar detection purposes. Since the pressure sensors are effective primarily for low frequency sound waves, and the motion sensors are effective primarily for high frequency sound waves, the invention is aggregately effective for a broad band spanning low and high frequencies.
U.S. Pat. No. 7,536,913, to Naluai, et al., issued May 26, 2009, discloses an underwater acoustic sensor designed for attachment to a rigid or semi-rigid mounting structure. The sensor includes an outer casing and a secondary casing spaced therefrom. A compliance layer is disposed between the inner surface of the outer casing and the outer surface of the secondary casing. An inner sensor support is designed to attach to the mounting structure and is spaced from the inner surface of the secondary casing. A plurality of sensor elements are disposed between and interconnect the inner surface of the secondary casing and the sensor support.
U.S. Pat. No. 7,633,206, to Andle, et al., issued Dec. 15, 2009, discloses reflective and slanted array channelized sensor arrays having a broadband source providing acoustic energy to a reflective or slanted array that reflects a portion of the incident signal to one or more sensing films wherein the response is measured.
The above patents and publications do not show discontinuities formed in the array that create replica waves having smaller wavelengths that are used to increase the directivity of the array. Those of skill will appreciate the present invention which provides improved directivity of a sonar array.